The present invention describes an operating mode for an internal combustion engine, in particular for a reciprocating piston engine, for example, for a gasoline engine with direct injection in a motor vehicle, having low-NOx combustion (NAV).
Downsizing can be used in the automotive engineering sector, in addition to other measures, in order to reduce CO2 emissions. In this context downsizing means constructing, employing and operating small-displacement engines in such a way that they achieve equivalent or better rankings with respect to driving behaviour when compared to their predecessor large-displacement engines. Downsizing allows fuel consumption to be reduced and thus CO2 emissions to be lowered. In addition, engines with smaller displacements have lower absolute frictional losses.
Smaller displacement engines are, however, characterised by having lower torque, especially at low speeds, leading to the vehicle having a poorer dynamic response and thus reduced flexibility. Disadvantages associated with the downsizing of gasoline engines can be largely compensated for through appropriate operating modes.
An operating mode is known from EP 1 543 228 B1 wherein, for example, a lean fuel/exhaust gas/air mixture in the combustion chamber of the internal combustion engine is caused to auto-ignite. In order that compression ignition occurs at the desired time, fuel is introduced into the lean, homogeneous fuel/exhaust gas/air mixture in the combustion chamber at the appropriate compression shortly before being spark ignited, so that a richer fuel-air mixture is formed embedded in the lean, homogeneous fuel/exhaust gas/air mixture. This rich fuel-air mixture serves as the initiator for compression-ignited combustion in the combustion chamber.
DE 10 2006 041 467 A1 contains a description for an operating mode for a gasoline engine having homogeneous compression-ignited combustion. If the homogeneous fuel/exhaust gas/air mixture, said mixture being a lean mixture, is compressed, in contrast to otto-cycle operating mode, combustion does not spread in the combustion chamber as a flame front originating from the point of ignition, but instead at an appropriate compression level the homogeneous fuel/exhaust gas/air mixture ignites at several points in the respective combustion chamber almost simultaneously, so that in this case controlled auto-ignition sets in. Controlled auto-ignition (RZV) exhibits significantly lower nitrogen oxide emissions along with high efficiency in terms of fuel consumption compared to the spark-ignition otto-cycle. This low-emission, efficient RZV operating mode with controlled auto-ignition can, however, only be used at a lower and possibly medium engine load/engine speed range, as knocking tendency increases with decreasing charge dilution, and thus the useful application of the RZV operating mode in higher engine load ranges is limited.
The present invention is concerned with the problem of specifying an improved, or at least an alternative operating mode for an, in particular directly injected, internal combustion engine, which is characterised in particular by reliable operating stability, inter alia in a higher engine load range with simultaneous low-NOx combustion.